Posted
by
Soulskill
on Saturday January 08, 2011 @06:18PM
from the marshmellowy-filling dept.

mapkinase writes with this excerpt from Discovery News:
"The Apollo Passive Seismic Experiment recorded motions of the ground from moonquakes and other activities generating sound waves until late 1977. The network was too limited to directly monitor waves bouncing off or scattered by the moon's core, leaving scientists dependent on more indirect techniques, such as measuring minute gravitational changes, to craft a picture of the moon's interior. Those models turned out to be pretty accurate, says lead scientist Renee Weber, with NASA's Marshall Space Flight Center in Huntsville, Ala. The new research confirms the existence of a solid inner core and liquid outer layer, similar to Earth's. Unlike Earth, the moon also has a partly melted, mushy layer over that."

Most Americans (most people in fact) would puke, if they saw how the "real" cheese made, and how it smells during the process.

I say this as an amateur cheese maker at home...

Eh, big deal. Not everybody has the stomach to watch a cow get slaughtered, or a keen interest in watching the process of sausage making, but that doesn't mean they don't enjoy the end result. I think it's a mistake to think that enjoying a food must mean a full appreciation and understanding of the intermediate steps that led to that food.

While I'm convinced that there is such a thing as American cheese, I understood that the generic term "american cheese" tends to refer to those cheese-colored slices of plastic Kraft et al provide. Am I wrong ?

Forgiven.1. Some theories said the moon had to be solid. It's smaller than Earth so it ought to have cooled faster. It has a lower average density than Earth so it shouldn't have lots of radioactive elements in its core, adding heat as they decay (Since all the long lasting radioactive isotopes are dense metals).2. You were probably informed that its calculated density showed the Moon couldn't have enough pressure near its center for an inner core to be crystaline iron, with an outer core of molten iron. That's what we think Earth is like. It explains our strong magnetic field, and its lack would explain why the Moon (and Mars, Mercury and Venus, also all somewhat smaller than Earth) doesn't (don't) have a similar magnetic field (s). That's only partly changed. This evidence suggests the moon has an inner core and outer core that are respectively solid and liquid (like Earth). It has a boundary layer above the outer core that goes gradually from liquid to slushy to sort of solid (unlike Earth, where the next boundary is pretty sharply defined). It has a solid crust (like Earth). So what's different besides that interesting slushyness? Iron. Earth's core is probably nearly all Iron, packed into a very regular crystal. Huge chunks of core have been pressurised enough to erase the irregularities between smaller crystals and merge them into one crystal structure wherever possible until you get to the top bit where it becomes more a bunch of discreet crystals and then molten Iron in the outer core. The Moon's core appears to be solid surrounded by liquid, but it doesn't appear to be almost all Iron - it still has much lighter material mixed in compared to Earth's core. So, if your high school geology teacher said the Moon couldn't have a solid Iron inner core with the vastest part of it in a regular crystal state, and a molten outer core, they may still have gotten it right, but if they went farther and said it couldn't be solid surrounded by liquid or couldn't be liquid at all, they definitely went too far in explaining the limited observations of the time.3. Some of the Selenologic data comes from Apollo. Some comes from more recent efforts like the south polar impactor mission. Not all that data matches, so it's probable this all needs more work and new instrumentation to be more confident we eventually get the whole model right. What's happened here is we have gotten closer to making the kind and quality of observations we have made to Earth itself during many earthquakes and other events, but arguably we are still not 100% caught up.

Thank you for the very informative post. I was just about post "if it has got a liquid core layer, where's the magnetic field" so thank you also for answering my stupid question before I got a chance to ask it.

Nice post. Just thought I'd point out one small mistake -- Mercury does have a magnetic field, despite its tiny size! Even though it is only 1% of the strength of Earth's, it envelops the entire planet and shields it from the solar wind, just like on Earth, and so much unlike the Moon, Mars and Venus.

Since it's closest to the sun Mercury is the most egg-shaped of the planets. The egg shape points toward the sun because the daylight side sees a larger gravitational field than the nighttime side; it gets pulled in harder. This effect is referred to as the tidal force; it falls off as the inverse cube power of the orbital radius. (Gravity itself only falls off as the inverse square, and extends further out.)

If a planet or moon has a tight orbit and rotates, tidal forces keep rearranging it internally all

Not exactly true... Mercury does indeed rotate with respect to the sun - 3 rotations for every 2 revolutions. It's rotation is tidally synchronous, but it is not tidally locked like our Moon is to Earth. So, while you may find a comfy spot at one of the poles, you certainly wouldn't want to head anywhere toward the middle.

Yeah I forgot about Mercury's 3:2 spin-orbit resonance that it picks up during eras of high orbital eccentricity induced by Earth and Venus. (Now the orbit is more circular, and we didn't know about the 3:2 thing for a long time because the sun is in our eyes.) A day on Mercury isn't infinitely long, but it's still two Mercury years.

This actually isn't an insurmountable problem. If you walked around Mercury's equator and always kept the sun on the horizon, you'd be moving an average of 2 mph. It sounds lik

Actually, all the long-lived radioactive elements (K-40, U, Th) are lithophile elements. That means they preferentially bond to silicic compounds. So, while U and Th are heavy metals, they aren't found in their elemental state. They form oxides and stuff and hang out in the mantle. There won't be much radioactivity in the core. Futhermore, these elements are among the first things to melt when mantle rocks are heated, so they preferentially go into the crust. So, if anything, we'd expect the moon to h

So if the moon started out with heavy elements like uranium, a lot of them will be in the core now, keeping it warm. The crust is mainly light stuff, silicon, etc, with the occasional lump of meteoric iron.

Incorrect. Elements segregate in the Earth (and Moon) based on chemical affinities [wikipedia.org], not on weight. And this is just relative abundance (relative to composition of the solar system). You get all elements in all parts of the Earth, but there is relatively more lithophile elements in the crust, and relatively more siderophile elements in the core.

And uranium is a lithophile, so it is more concentrated in the crust. It still keeps the core warm though. The crust is like an electric blanket, it insulates and provides heat (through radioactivity) to the core (and mantle).

So what happens if or when we mine enough Uranium from the Earth? Would the drop in radioactive heat allow the core to cool significantly faster, or is it just a redundant heat source? I'm working on the assumption that, even if we did mine out all the Uranium, the core wouldn't cool down fast enough to matter to anyone with an average human life span, but all the same I'm curious just how much of a cooling impact there would be.

To correct/or add to my previous point, most of the heat generated would be from the mantle and core, not the crust. Even though uranium etc are more concentrated in the crust, the much higher volume of the mantle/core negates this. Also, other elements provide heat through radioactivity, such as thorium and potassium-40.

Finally, we can't mine all the uranium. It's only profitable to mine highly concentrated uranium, close to the surface. How concentrated or deep will change in the future as demand increase

Incorrect. Elements segregate in the Earth (and Moon) based on chemical affinities, not on weight.

The mantle is below us and mostly made out of heavy elements, or at least it contains a higher concentration thereof. It's probably safe to say that elements segregate both due to weight and chemical affinities. It's not like uranium is going to jump up off the soil if you hold some rocks over it. This characteristic is more produced in the mantle than the crust, probably due to melting effects.

And somewhere in between the surface and the core is a temperate zone where water is in its liquid phase. Since we have found life in every environment on Earth where water is liquid, we need to assume that there is some kind of thermal driven ecosystem inside the Moon.

Are there selenites? Possibly so. I hope the lunar exogeolists are talking with the biologists who have been studying our black smokers.

Check out the temperature at 2 metres depth [nasa.gov]. I reckon your temperate zone is close enough to the surface that the regolith at that depth will be as dry as it is at the surface (except in cold polar craters).

Conclusion: other than at the pole the moon may be too hot and dry for life as we know it.

I do not understand the parent comment, and do not see the relevance of the reference provided there.

The closest I got was in Chapter 5 of the reference where Apollo 17 found a temperature close to 256 K at a depth of 2 meters [nasa.gov] (scan down the page to the second chart insert). This is less than 2 degrees Fahrenheit, and at that shallow depth is probably around the average annual temperature of the surface, as in Earth caves. There is no reason not to expect the temperature to continue to decrease for many m

Sorry I should have been more specific. I was looking at Figure 06 which shows temperature increasing at roughly 0.02 degrees C per centimetre of depth. Another 40 degrees gets you to 300 Kelvin at ~20 metres depth. But the moon is pretty much gardened to that depth in the sense that meteors dig craters that deep and turn the surface over. So if there was water at 20 metres at one time it is unlikely to be there now, except at the poles which are very cold.

it looks artificial like no other thing in the solar system does. so much that that many asteroids hitting over all those aeons only had had created that many impact and changed its landscape only so much. absurdly, its uniform gray dust.

Looks a lot like other moons, and even like Mercury, and not totally unlike Mars.

Its the only moon in a warn (not hot, not frozen) zone, and its far from uniform.

If it was totally solid you might expect more landscape features created by impact. But because it is simi-fluid and reasonably large, gravitational forces keep super-large scale features from being formed.

incorrect. mercury and mars, have varying atmospheric or environmental conditions shaping them. there is a reason why they are that flat, and uniform. there is something grinding the stones to sand.

you have easily accepted the fluidity that was just a new proposition. it is equally interesting that you havent asked why the moon was already that fine grained up till this point. it is as if it was custom ordered to perfectly absorb meteorites, being not too soft, or not too hard, and finely grained. there

Very interesting point when in considering that in proportion to it host planet (huge ration actually!!), its the largest moon in sol, and as thus, having the most influence over its host planet, with the most important force at play with bodies this big - gravity.

incorrect. mercury and mars, have varying atmospheric or environmental conditions shaping them. there is a reason why they are that flat, and uniform.

Mars is flat?? I don't know where you get that idea from. Mars has mountains and valleys that dwarf anything we have on earth. Olympus Mons is over 21km tall, almost 3 times the height of anything on earth.

Mars is flat?? I don't know where you get that idea from. Mars has mountains and valleys that dwarf anything we have on earth. Olympus Mons is over 21km tall, almost 3 times the height of anything on earth.

Maybe he's basing it on some of the pictures that have been sent back (though obviously not the ones of inside a crater) which have mostly been from fairly flat locations as they're easier to land safely in and get good scientific results back from.

flat.
it has mountains and valleys that dwarf anything here on earth, however it doesnt have huge oceans covering huge depths, or huge mountain ranges that go half a continent, like here. its appalling that you talk about mountains and valleys yet forget huge oceans that have 11,000 m as their deepest point in a hole that covers almost half of the planet on one side, not to mention others in other oceans. its not just a mountain, it is a huge inward landscape on all sides of the planet, and outer protrusio

it has mountains and valleys that dwarf anything here on earth, however it doesnt have huge oceans covering huge depths, or huge mountain ranges that go half a continent, like here. its appalling that you talk about mountains and valleys yet forget huge oceans that have 11,000 m as their deepest point in a hole that covers almost half of the planet on one side, not to mention others in other oceans. its not just a mountain, it is a huge inward landscape on all sides of the planet, and outer protrusion on other. take oceans off of the earth in your mind's eye, then rethink.

The height of Olympus Mons I have is height above Datum. Mars has deep basins that go far below datum also.

From Wikipedia:

"Since Mars has no oceans and hence no 'sea level', it is convenient to define an arbitrary zero-elevation level or "datum" for mapping the surface. The datum for Mars is defined in terms of the height at which the air has a particular pressure at about the freezing point of water: a pressure of 610.5 Pa (6.105 mbar), approximately 0.6% of Earth's, at a temperature of 273.16 K. This pre

mercury and mars, have varying atmospheric or environmental conditions shaping them. there is a reason why they are that flat, and uniform. there is something grinding the stones to sand.

The temperature difference between night and day grind moonstones to dust. So do the very meteor collisions you mentioned. And lack of tectonic or volcanic activity means there's no new mountains being rised, so of course the end result is a flat, dust-filled world.

its so easy to justify. thats the problem of contemporary science. very easy to produce half assed explanations for everything.

that half assed one, for example, fails on the face of the fact that there are even greater forces, be it temperature difference, or others, acting on other planets and moons. yet no planet is such finely grained into sand.

Many of the other large bodies is space are more mountainous and uneven specifically because of the fact that they have much greater forces applied to them on a regular basis. Those forces are not evenly distributed across the planet, and cause mountains to rise from tectonic pressure and valleys to be carved from 100km/h sandstorms. The moon has no volcanic activity to create new mountains, and no atmosphere/rain to erode valleys, and as such the only real surface details are the overlapping craters left

there is a reason why they are that flat, and uniform. there is something grinding the stones to sand.

The variation in a planet's crust (from mountains to trenches) are tiny compared to it's diameter (less than a percent).
Sure atmospheric conditions contribute a bit by grinding down mountains a mile or two, but most of the flatness of rocky planets is caused by gravity and hydrostatic equilibrium.
Meaning that molten rock tried to form a spheroid (a bit bulgy at the equator due to rotational forces), simi

There have been discussions of a probe to Uranus [nationalgeographic.com].
Don't the Slashdot editors realize how many more silly jokes and pageviews this could generate? As a stockholder in GKNT, I demand that you post
a story about the Uranus probe.

But then, if the moon really is formed out of stuff from earth (which contains a lot of iron ore), and it *does* have a liquid core (making that iron spin) - then why does it not have a proper magnetic field ? Is its rotation too slow ?

It's not enough for the iron to be molten. The fluid also has to be moving turbulently to generate a magnetic field. This is accomplished either by the core being cooled from above or by the core freezing at the center. If it's not cooling fast enough, you don't get dynamo activity.

I suppose a bit of heat comes from being swung around in the sun's gravitational field, but the crust would feel that more than the core. I don't see why there can't be a lot of radioactive material in the core. Additionally the Apollo heat flow experiment should have shown how much heat is actually being lost through the crust. Maybe the core hasn't had enough time to cool entirely. Its a pretty small core.

Well yeah. Its because the moon is in orbit around the sun and the moon at the same time. The orbit has to be fairly eccentric around both so the changing intensity of both fields raises small tides in the lunar crust.

I suppose its part of the picture. I read somewhere that the core is offset towards the Earth by about 1 km. The moon does wobble slightly. Telescopes on Earth can see about 60% of the lunar surface by observing at the right times. The sun will be continually pulling at the moon to point towards it.

Consider Janus and Epimetheus [wikipedia.org]. Do they orbit each other, or do they orbit Saturn? Both statements are true. This is also the case for the Moon and the Earth. Its really a difference of degree.

What about the Earth's gravitational field? Wouldn't that have a significant effect as well?

I think the point was that it's going around the sun so it has sunlight shining on it. IANAA but I don't believe the Moon is subject to any significant 'kneading' like Titan; I imagine this is because the Moon is so large compared to the planet it's orbiting.

What about the same reason that keeps the inners of the Earth warm : nuclear fission ? After all, the kaguya probe has found decent amounts of it on the moon's surface (one of the most underrated science news of the recent times, imho) so it is not far fetched to imagine it containing a decent proportion near its core. Especially if it is liquid.

All else equal, sure. But if one of those layers is insulative (at least compared to the larger neighbors) then all bets are off. The Moon is known to be less dense than the Earth, so you would expect the rate of thermal transfer to be less.

My understanding is that it still has radioactives. Also there's modest tidal forces acting on the Moon (mostly from the Sun). I guess the last heat source would be heat of crystallization of the liquid core. I'm a bit surprised that the core is still liquid. It must be that the Moon is a great thermal insulator right now.

Reminds me that Astronauts working on the surface had to keep their suits as clean as possible to help stay cool. Partly because being dark means you collect more heat from the sun but also because the dust is a good insulator. If the crust is made of the same stuff there won't be a lot of heat flowing through it. I also read that the daytime heat on the moon is gone one metre below the surface.

I also read that the daytime heat on the moon is gone one metre below the surface.

Keep in mind that daytime heat has only two weeks to penetrate before it is replaced with some serious nighttime cooling. You wouldn't have to go far down before the heating and cooling tend to cancel each other out.

If the sun was exerting a greater tidal effect then how come it keeps the same side facing Earth?

The sun doesn't and your observation is correct. But for heating purposes, the Moon is almost static with respect to the Earth. There's a little rocking, but nothing significant. On the other hand, the Moon's orientation with respect to the Sun is still effectively a rotation every four weeks. So even through the Earth's tidal force is much stronger, the Solar tidal forces probably contribute more to internal heating. I could be wrong about this since I haven't given it a great deal of thought, and the Eart

It's obvious that the OP was talking about compression on the moon from tidal forces, not just tidal force. Since the moon is in tidal lock with the earth this is basically zero for earth. And since tidal forces actually follow an inverse-cube law, not inverse-square, the tidal compression from the sun is not really worth mention either.

Tidal forces from the Sun upon the Earth are sufficient to be noticed and a significant factor when calculating what the daily high tide is going to be, and its impact can usually be measured on the order meters in tidal height on a daily basis. I consider that to be substantial.

The Moon is for all practical purposes at the same distance, although a bit smaller so the tidal forces from the Sun will be reduced... not due to the distance from the Sun itself but rather the sheer size of the Moon. And the Moo

IIRC, there's still some debate as to how much of the interior heat of the Earth is due to radioactive decay and how much is residual heat leftover from the planet's formation. Remember that the Earth/Moon system originated as two bodies of similar mass that collided a few billion years back; both would have been fully molten, surface to core, when the proverbial dust had settled. Millennia would pass before either had a solid surface.

It might be that the internal heat of the Earth is partly residual, with radioactive decay delaying cooling by adding more heat. Regardless of the proportion of residual to radioactive heat, the moon should be less molten than the Earth, if only due to the square-cube law dictating the Earth will cool more slowly. So the science in TFA is actually pretty interesting.

Before the discovery of radioactivity it was estimated that the Earth would only take 20MA to reach its current temperature from a fully molten state. So the internal heat is almost entirely radioactive, meaning that the degree of molten-ness depends mostly on composition, not square-cubiness.
Also, exotic is when you use an endangered macaw not a chicken.

Once the outer surface has solidified, it insulates the core quite well. The moon's surface temperature drops to 100K (almost cold enough to liquify oxygen in Earth's atmosphere) in the night, which is pretty cold considering that the sun heats it to 390K (hotter than boiling water) by day. In other words, the surface doesn't get much heat from below. A few billion years might just not be enough for it to cool out completely.